Illustrating the effect of Earth curvature and atmospheric refration correction in viewshed calculations

By toggling on and off the two viewshed layerss on the map below, you can compare the results of using the terra::viewshed() with Earth curvature and atmospheric refraction correction disabled, and terra::viewshed() with correction enabled using the default co-efficient of 0.85714.

The calculated viewsheds are for a mythical 10m high tower viewpoint near the centre of Cobar in western NSW, where the terrain is quite flat.

It is easy to see that without the correction, the calculated viewshed is much larger because the curvature of the Earth is ignored (although refraction by the atmossphere counteracts the effects of curvature, but only partially).

This is the first step in developing documented, reproducible, open-source capabilities for visual impact assessment of large structures such as wind turbines. The intention is to implement many of the metrics and enhancements to simple viewsheds described in this paper.